Preheat the oven to 350°F (175°C) and grease a 9x13-inch pan.
In a medium bowl, whisk together flour, baking powder, and salt; set aside.
In a large bowl, cream butter and sugar until light and fluffy, about 3–4 minutes.
Beat in eggs one at a time, followed by vanilla extract, until smooth.
Fold in the dry ingredients alternately with sour cream until fully combined; batter should be thick but pourable.
Gently fold in chocolate chips and nuts, leaving a few for sprinkling on top.
Spread batter evenly in the prepared pan and sprinkle remaining chocolate chips and nuts on top.
Bake 30–35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Let cool in the pan for 10 minutes, then cut into squares while still slightly warm for that gooey texture.
